Ignore comment-only build script (#754)

Sometimes LLM suggests a build script with comments-only when it wants
to reuse the existing build script, e.g.,
```
<build script>
# Reuse the existing build.sh
</build script>
```

This avoids confusing LLM to 'think' the build script works.
Returning `''` will make OFG use the existing build script.

agent/base_agent.py

@@ -75,6 +75,9 @@ def _filter_code(self, raw_code_block: str) -> str:
         line for line in raw_code_block.splitlines()
         if not line.strip().startswith('```')
     ]
+    # Sometimes LLM returns a build script containing only comments.
+    if all(line.strip().startswith('#') for line in filtered_lines):
+      return ''
     filtered_code_block = '\n'.join(filtered_lines)
     return filtered_code_block